Light emitter with a conductive transparent p-type layer structure

ABSTRACT

A light emitting device includes an n-type layer, a p-type layer structure, a layer of p-type nano-dots imbedded in the p-type layer structure, and an active region sandwiched between the n-type layer and the p-type layer structure, where the p-type nano-dots possess a sheet density of 10 10  to 10 12  cm −2 , a lateral dimension of 2-20 nm, and a vertical dimension of 1-5 nm. The p-type layer structure with a layer of p-type nano-dots imbedded therein provides good vertical conductivity and UV transparency. Also provided is a method for making the light emitting device.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of, and claims the prioritybenefit of, U.S. application Ser. No. 14/687,886 filed on Apr. 15, 2015.

1. FIELD OF THE INVENTION

The present invention relates in general to semiconductor light emitterswith a conductive transparent p-type layer structure, more particularlyto group III nitride compound semiconductor ultraviolet light emitterswith a conductive transparent p-type AlGaN layer structure with a layerof p-type nano-dots imbedded therein, and method of forming the same.

2. DESCRIPTION OF THE RELATED ART

Nitride compound semiconductors such as InN, GaN, AlN, and their ternaryand quaternary alloys are viewed as very important optoelectronicmaterials. Depending on alloy composition, nitride compounds can enableultraviolet (UV) emissions ranging from 410 nm down to approximately 200nm. This includes UVA (400-315 nm), UVB (315-280 nm), and part of UVC(280-200 nm) regimes. UVA emissions are leading to revolutions in curingindustry, and UVB and UVC emissions owing to their germicidal effect arelooking forward to general adoption in food, water, and surfacedisinfection businesses. Compared to the traditional UV light sources,such as mercury lamps, UV light emitters made of nitride compounds offerintrinsic merits. In general, nitride UV emitters are robust, compact,spectrum adjustable, and environmentally friendly. They offer high UVlight intensity & dosage, which is ideal treatment for fresh food, waterand surface storage, disinfection, and sterilization. Further, the lightoutput can be modulated at frequencies up to a few hundreds ofmega-hertz, promising them innovative light sources for covertcommunication and bio-chemical detection.

At the present, commercially available UVB and UVC light-emitting diodes(LEDs) commonly adopt the laminate structure containing a c-planesapphire as UV transparent substrate, an AlN layer coated over thesubstrate serving as epitaxy template, and a set of AlN/AlGaNsuperlattice for dislocation and strain management. The utilization ofAlN template and AlN/AlGaN superlattice enables the growth ofhigh-quality high-conductivity n-type AlGaN electron supplier layer,which injects electrons into the following AlGaN-based multiple quantumwell (MQW) active-region. On the other side of the MQW active-region arean AlGaN electron-blocking layer, an AlGaN hole injection layer, a holesupplier layer and a p-type GaN or InGaN layer for ohmic contactformation. The prior art AlGaN UV LED structures can be found in thereference. (e.g., “Milliwatt power deep ultraviolet light-emittingdiodes over sapphire with emission at 278 nm”, J. P. Zhang, et al,APPLIED PHYSICS LETTERS 81, 4910 (2002), the content of which isincorporated herein by reference in its entirety.).

Since acceptor ionization energy increases linearly with Al-compositionin AlGaN material, and hole concentration decreases exponentially withacceptor ionization energy, acceptor-doped AlGaN material (p-type AlGaNor p-AlGaN) possesses exponentially increasing electrical resistivitywith Al-composition. This means conventional p-type AlGaN layers whichare UV transparent usually are highly resistive, unsuitable for servingas a hole supplier layer and p-type ohmic contact layer. Hence in theprior art UV LEDs p-type GaN or InGaN layer is commonly used as holesupplier layer and ohmic contact layer. P-type GaN and InGaN layers areUV opaque and the application of them in UV LEDs severely limits the LEDlight extraction efficiency. The prior art UV LEDs emitting in the UVBand UVC regimes practically only have a light extraction efficiency assmall as 6%-10%.

Superlattice structures containing p-type AlGaN barrier layers andp-type GaN well layers have been proposed to replace conventional p-typeAlGaN layer for improved conductivity and preserved UV transparency(e.g. U.S. Pat. Nos. 5,831,277, 6,104,039, and 8,426,225, the contentsof which are incorporated herein by reference in their entirety). Thevalence band and polarization discontinuities between AlGaN and GaN willlead to hole accumulation within the GaN wells. Holes can move freelywithin the GaN well planes, however, the AlGaN/GaN valence band andpolarization discontinuities will impede hole movement in the directionsperpendicular to the GaN well plane. This is to say, that the p-typeAlGaN/GaN superlattice can have improved lateral conductivity yet withlimited vertical conductivity, not suitable for vertical hole injectioninto the MQW active-region for light emissions. To enhance the verticalconductivity of the p-AlGaN/GaN superlattice, the thickness of thep-AlGaN barrier layer within the superlattice can be estimated accordingto hole's Bohr radius as the rule of thumb:

$a_{B} = {\frac{4{\pi ɛ}_{r}ɛ_{0}h^{2}}{m_{k}e^{2}} = {{\frac{m_{0}}{m_{h}}ɛ_{r}a_{B\; 0}} = {0.529\frac{m_{0}}{m_{h}}{ɛ_{r}(Å)}}}}$

since the hole's effective mass m_(h) in AlGaN is very heavy, close tothat of the free electron mass, m₀, and the relative permittivity ε_(r)of AlGaN material is in-between of 8 to 9 depending on Al-composition,hole's Bohr radius within AlGaN is approximately around 5 Å. Whenapplying such a thin AlGaN layer in the AlGaN/GaN superlattice, a) ifthe GaN well layer is thick enough to maintain a good AlGaN/GaNinterface, the superlattice will be UV opaque; b) if the GaN well layeralso maintains the ultrathin thickness for UV transparency, theultrathin AlGaN/GaN superlattice interface will be vanishing because ofinterface roughness and composition mixing, which turns the ultrathinperiod AlGaN/GaN superlattice identically to a conventional AlGaN alloy,losing all the hole accumulation benefit.

The present invention discloses p-type AlGaN structures with improvedvertical hole conductivity yet maintaining UV transparency, and UV LEDswith such p-AlGaN structures with improved light extraction efficiency.

3. SUMMARY OF THE INVENTION

One aspect of the present invention relates to a light emitting device,which includes an n-type layer, a p-type layer structure, a layer ofp-type nano-dots imbedded in the p-type layer structure, and an activeregion sandwiched between the n-type layer and the p-type layerstructure.

The p-type nano-dots may have a sheet density of 10¹⁰ to 10¹² cm⁻², alateral dimension of 2-20 nm, and a vertical dimension of 1-5 nm.

The p-type layer structure may include a smooth p-type layer in directcontact with the active region, a rough p-type layer formed on thesmooth p-type layer and having protrusions and depressions formedbetween the protrusions, and a confining p-type layer, where the layerof p-type nano-dots is formed on the rough p-type layer with the p-typenano-dots being formed in the depressions of the rough p-type layer, theconfining p-type layer is conformally formed on the layer of p-typenano-dots.

The p-type layer structure may include a plurality of layers of thep-type nano-dots and a plurality of the confining p-type layersalternately and conformally stacked with the layers of the p-typenano-dots. The p-type nano-dots in the plurality of layers of the p-typenano-dots can be substantially vertically aligned.

The smooth p-type layer, the rough p-type layer, and the confiningp-type layer can be made of AlGaN, with Al-composition of the smoothp-type layer being equal to or higher than Al-composition of the roughp-type layer, and Al-composition of the confining p-type layer beingsubstantially equal to the Al-composition of the rough p-type layer, andthe Al-composition of the rough p-type layer can be higher than 40%.

The p-type nano-dots can be made of GaN or InGaN with In-compositionless than 10%, or AlGaN with Al-composition less than 5%.

The smooth p-type layer may have a thickness in the range of 5-30 nm,the confining p-type layer may have a thickness in the range of 2-10 nm.

The rough p-type layer may have a base, the protrusions are formed onthe base, the base may have a thickness in the range of 0-10 nm, and theprotrusions may have a sheet density of 10¹⁰ to 10¹² cm⁻², a lateraldimension of 10-100 nm, and a vertical dimension of 5-30 nm.

The layer of p-type nano-dots can be a discontinuous or continuouslayer.

In another embodiment, the p-type layer structure includes a smoothp-type layer in direct contact with the active region, a first smoothconfining p-type layer formed on the smooth p-type layer, and a secondsmooth confining p-type layer, where the layer of p-type nano-dots isformed on the first smooth confining p-type layer, and the second smoothconfining p-type layer is formed on the layer of p-type nano-dots.

The smooth p-type layer, the first smooth confining p-type layer, andthe second smooth confining p-type layer can be made of AlGaN, withAl-composition of the smooth p-type layer being 5% higher thanAl-composition of the first smooth confining p-type layer, andAl-composition of the second smooth confining p-type layer beingsubstantially equal to the Al-composition of the first smooth confiningp-type layer, and wherein the Al-composition of the first smoothconfining p-type layer is higher than 30%.

The layer of p-type nano-dots can be made of InGaN with In-compositionin the range of 0-10%, or AlGaN with Al-composition in the range of0-5%, inclusively, so that the compressive strain experienced by thelayer of p-type nano-dots is larger than 0.7% during epitaxial growth onthe first smooth confining p-type layer.

The p-type layer structure may include a plurality of layers of p-typenano-dots and a plurality of second smooth confining p-type layersalternately stacked with the layers of p-type nano-dots.

The layer of p-type nano-dots is a continuous layer including wettinglayer portion laterally surrounding the p-type nano-dots, and athickness of the wetting layer portion can be in the range of 0.6-1.0nm.

In another embodiment, the p-type layer structure may include a smoothp-type layer in direct contact with the active region and a smoothconfining p-type layer, the layer of p-type nano-dots is formed directlyon the smooth p-type layer, and the smooth confining p-type layer isformed on the layer of p-type nano-dots.

The light emitting device may further include a contact pad formeddirectly on the p-layer structure.

The light emitting device may further include a p-type GaN or InGaNohmic contact layer formed on the p-layer structure and with a thicknessof less than 10 nm, such as 2-8 nm.

Another aspect of the present invention relates to a method of forming avertically conductive UV transparent p-type layer structure, whichincludes:

forming a smooth p-type AlGaN layer over a substrate at a temperaturebetween 1000-1050° C.;

forming a rough p-type AlGaN layer with surface protrusions of height 5to 30 nm, lateral size 10 to 100 nm, and sheet density 10¹⁰ to 10¹² cm⁻²on the smooth p-type AlGaN layer at a temperature 50-200° C. lower thanthat of the smooth p-type AlGaN layer;

forming a p-type GaN or InGaN layer on the rough p-type AlGaN layer suchthat nano-dots are formed within depressions between the protrusions ofthe rough p-type AlGaN layer, wherein the nano-dots possess a sheetdensity of 10¹⁰ to 10¹² cm⁻², a lateral dimension of 2-20 nm, a verticaldimension of 1-5 nm; and

conformally forming another p-type AlGaN layer on the p-type GaN orInGaN layer to cover the nano-dots.

The formation of the rough surface p-type AlGaN can be performed attemperatures in the range of 800-1000° C.

Another aspect of the present invention relates to a method of forming avertically conductive UV transparent p-type structure, which includes:

forming a p-type AlGaN layer with Al-composition larger than 30% at atemperature of 1000-1050° C.;

forming a p-type GaN or InGaN material with nominal thickness of 1-5 nmat a temperature of 800-1000° C.

The step of forming the p-type AlGaN layer and the p-type GaN or InGaNmaterial can be repeatedly alternatively performed 3-10 times.

4. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and constitute a part of thisapplication, illustrate embodiments of the invention and together withthe description serve to explain the principle of the invention. Likereference numbers in the figures refer to like elements throughout, anda layer can refer to a group of layers associated with the samefunction.

FIG. 1A illustrates the cross sectional schematic view of a partialp-type layer structure according to an embodiment of the presentinvention.

FIG. 1B illustrates a bird's eye view of the partial p-type layerstructure shown in FIG. 1A.

FIG. 2 illustrates the cross sectional schematic view of a conductive UVtransparent p-type layer structure based on the partial p-type layerstructure shown in FIG. 1.

FIG. 3 illustrates a LED structure employing a transparent conductivep-type layer structure according to an embodiment of the presentinvention.

FIG. 4 illustrates the cross sectional schematic view of a conductive UVtransparent p-type layer structure according to another embodiment ofthe present invention.

FIG. 5 illustrates a LED structure employing the transparent conductivep-type layer structure according to another embodiment of the presentinvention.

5. DETAILED DESCRIPTION OF EMBODIMENTS

The present invention discloses a light emitting device with improvedlight extraction efficiency. Throughout the specification, the termIII-nitride or nitride in general refers to metal nitride with cationsselecting from group IIIA of the periodic table of the elements. That isto say, III-nitride includes MN, GaN, InN and their ternary (AlGaN,InGaN, InAlN) and quaternary (AlInGaN) alloys. In this specification, aquaternary can be reduced to a ternary for simplicity if one of thegroup III elements is significantly small. For example, if theIn-composition in a quaternary AlInGaN is significantly small, smallerthan 1%, then this AlInGaN quaternary can be shown as ternary AlGaN forsimplicity. Using the same logic, a ternary can be reduced to a binaryfor simplicity if one of the group III elements is significantly small.For example, if the In-composition in a ternary InGaN is significantlysmall, smaller than 1%, then this InGaN ternary can be shown as binaryGaN for simplicity. III-nitride or nitride can also include smallcompositions of transition metal nitride such as TiN, ZrN, HfN withmolar fraction not larger than 10%. For example, III-nitride or nitridemay include Al_(x)In_(y)Ga_(z)Ti_((1-x-y-z))N,Al_(x)In_(y)Ga_(z)Zr_((1-x-y-z))N, Al_(x)In_(y)Ga_(z)Hf_((1-x-y-z))N,with (1−x−y−z)≦10%. A III-nitride layer or active-region means that thelayer or active-region is made of III-nitride semiconductors.

In the following contents, wurtzite c-plane nitride light-emittingdevices or structures are used as examples to elucidate the principleand spirit of the present invention. The teachings in this specificationand given by the following embodiments can be applied to non-c-planenitride semiconductors, II-VI semiconductors and other semiconductordevices.

According to embodiments of the present invention, a conductiveUV-transparent p-type layer structure is used in a light emittingdevice, such as a UV LED. The conductive UV-transparent p-type layerstructure can be used as an electron-blocking layer, a hole injectionlayer, a hole supplier layer, or an ohmic contact layer, or function asa combination of two or more of these layers because of its excellentvertical conductivity and UV-transparency. The p-type layer structureincludes one or more layers of p-type nano-dots imbedded in the p-typelayer structure. For example, the p-type layer structure may include twop-type layers such as two p-type AlGaN layers with one layer of p-typenano-dots sandwiched therebetween, or include two or more p-type layersand two or more layers of p-type nano-dots with each layer of p-typenano-dots sandwiched between two of the p-type layers. The p-type layerscan be the same or different. For example, when using p-type AlGaNlayers, the Al-composition of these p-type AlGaN layers can be the sameor different. In one embodiment, the Al-composition of the p-type AlGaNlayer in direct contact with the active region is 5-10% higher than thatof the rest p-type AlGaN layers so as to provide good electron blockingeffect. The layer of p-type nano-dots can be formed via epitaxial growthon a rough surface of a p-type layer, or via epitaxial growth on asmooth surface of a p-type layer based on biaxial compressive strain, orvia any other suitable method.

The layer of p-type nano-dots can be made of GaN or InGaN withIn-composition less than 10% such as 3-7%, or AlGaN with Al-compositionless than 5%. The p-type nano-dots may have a sheet density of 10¹⁰ to10¹² cm⁻² such as 10¹¹ to 10¹² cm⁻², a lateral dimension of 2-20 nm suchas 5-15 nm, and a vertical dimension of 1-5 nm such as 1-2.5 nm. Thelayer of p-type nano-dots can be a continuous layer including thin layerportion connecting the p-type nano-dots, and the thickness of the thinlayer portion can be in the range of 0.3-1.0 nm when the layer of p-typenano-dots is formed on a rough surface of a p-type layer, and thethickness of the thin layer portion can be in the range of 0.6-1.0 nmwhen the layer of p-type nano-dots is formed on a smooth surface of ap-type layer via large biaxial strain (the thin layer portion is alsoreferred to as wetting layer portion in this case.).

Illustrated in FIG. 1A is the cross sectional schematic view of apartial p-type layer structure according to an embodiment of the presentinvention, which includes a smooth p-AlGaN layer 41 and a rough p-AlGaNlayer 42, with p-type conductivity achieved via Mg-doping. TheAl-compositions of layers 41 and 42 can be higher than 40%, in order tobe UV transparent for the wavelengths below 320 nm. And theAl-composition in layer 41 can be higher than that of layer 42, forexample, by 5-10%. In this specification, a smooth or rough layer meansthat the surface of the layer is of nanometer-scale smoothness orroughness. Specifically, a smooth or rough layer can be identified byits surface root mean square (RMS) of roughness. When evaluated by asurface scan metrology, such as atomic force microscope (AFM), if thesurface RMS of roughness is less than 3 nm, the layer is a smooth layer;otherwise, it is a rough layer.

Layer 41 can be formed directly on an active-region, with a thickness inthe range of 5-30 nm. For example, layer 41 is formed on thelight-emitting active-region 30 of a light emitting device such as a UVlight emitting device, serving as an electron blocking layer, as shownin FIG. 3. According to an embodiment of the present inventions, layer42 formed on layer 41 possesses substantial surface roughness. Layer 42has a base 420 and many surface protrusions 421. Base 420 can have apredetermined thickness (e.g. 2-10 nm, or 5-10 nm) or have a vanishingthickness (i.e. base 420 vanishes), and surface protrusions 421 sittingon base 420 are separated or laterally surrounded by surface depressions422 (or sitting on layer 41 when base 420 vanishes). For clarity, FIG.1B illustrates one possible bird's eye view of layer 42 shown in FIG.1A. The lateral dimension, D, measured for the largest lateral dimensionof protrusions 421, is in the range of 10 to 100 nm, for example 20 to50 nm. This translates into a sheet density of surface protrusions 421and depressions 422 approximately 10¹⁰ to 10¹² cm⁻². The protrusionheight, or depression depth, H, is in the range of 5 to 30 nm, forexample 10 to 20 nm. Protrusions 421 and depressions 422 are formed vialow-temperature epitaxial growth. That is to say, the epitaxialformation temperature of layer 42 is much lower than that of layer 41.According to one embodiment of the present invention, metalorganicchemical vapor deposition (MOCVD) is used to epitaxially form layers 41and 42, where the formation temperature of layer 41 is typically within1000-1050° C., and the formation temperature of layer 42 is typically50-200° C. lower, i.e., within 800-1000° C. The lower the formationtemperature, the higher the surface protrusions/depressions density.According to the present invention, Mg-doped p-type AlGaN requiressubstantial growth temperature to maintain smooth surfacetwo-dimensional growth mode. At lower temperatures, the increased Mgincorporation rate and reduced Al-adatom surface mobility change thegrowth mode into three-dimensional island growth, resulting in surfaceprotrusions and depressions. To further facilitate surfaceprotrusions/depressions formation, the formation ambient can be switchedfrom ammonia-hydrogen ambient to ammonia-nitrogen ambient.

The high sheet density of protrusions 421 and depression 422 is desiredto formation of vertically conductive and UV transparent p-type AlGaNlayer structures. FIG. 2 illustrates the cross-sectional schematic viewof a vertically conductive UV transparent p-type AlGaN layer structure40 according to an embodiment of the present invention. Directly formedon the rough surface of layer 42 is a nominal layer 43. Layer 43 can bea Mg-doped GaN layer, or InGaN layer with In-composition of 0-10% suchas 3-6%, or AlGaN layer with significantly smaller Al-composition ascompared to that of layer 42, such as 0-10%. In one embodiment, nominallayer 43 is made of Mg-doped GaN. Nominal layer 43 contains portions 432formed on facets or sidewalls of protrusions 421 and portions 431 formedin the bottom parts of depressions 422. The facets or sidewalls ofprotrusions 421, because of the higher surface formation energy, permitmuch less formation of layer 43. As a result, layer 43 is predominantlyformed within the bottom parts of depressions 422 and constitutes alayer of p-type nano-dots. That is to say, portions 432 can have muchless or vanishing thickness as compared to portions 431. Therefore,layer 43 (i.e., the layer of p-type nano-dots) can be a continuous layerwhere the whole rough surface of layer 42 is covered by layer 43, or adiscontinuous layer where a portion of protrusions 421 is exposed bylayer 43. In case of a discontinuous layer, no layer 43 grows on top orupper portions of sidewalls of at least some protrusions 421, forexample, 20-90% or 40-70% of the protrusions 421. Formed on nominallayer 43 is layer 44, which is a Mg-doped AlGaN layer withAl-composition equal or close to that of layer 42. The formationconditions of layer 43 and 44 are different from those for layer 42 butidentical or close to those for layer 41. Because of thehigh-Al-composition, layer 44 is conformably formed over layer 43, witha rather uniform thickness of 2-10 nm. Layers 42, 43 and 44 are soformed that portions 431 are of a lateral dimension, d, in the range of2-20 nm such as 5-15 nm, and a vertical dimension, h, in the range of1-5 nm such as 1-3 nm. That is to say, portions 431 arethree-dimensionally confined nano-dots. Compared to the onedimensionally confined GaN well layer in the prior art p-type AlGaN/GaNsuperlattice, portions 431 being three-dimensionally confined possessthree dimensional polarization and valence band discontinuities henceaccumulate much higher concentration of free holes. In the prior artp-type AlGaN/GaN superlattice, hole concentration in the GaN well layercan be as high as 10¹⁸ cm⁻³. According to this invention, because of thestronger confinement and more polarization discontinuities, the holeconcentration within portions 431 can be in the range of 10¹⁹-10²⁰ cm⁻³.

Further refer to FIG. 2, layer 43 and layer 44 can be repeatedly stackedon top of each other for more than one time. For example, layers 43/44can be stacked for 3 times to 10 times. Repeatedly alternativelystacking layer 43 and layer 44 leads to formation of arrays ofvertically aligned nano-dot portions 431. When a vertical forward biasvoltage is applied to p-AlGaN layer structure 40, the otherwiseuniformly distributed electric field will be redistributed, intensifiedand focused on the highly conductive nano-dot portions 431, enablinghole tunneling current within the vertically aligned highly conductivenano-dot portions 431. That is to say, p-AlGaN layer structure 40 is avertically conductive UV transparent p-type AlGaN layer structure.

FIG. 3 illustrates a UV light emitter device structure employed withvertically conductive UV transparent p-AlGaN layer structure 40.Substrate 10 can be selected from sapphire, AlN, SiC, Si, and the like.Formed over substrate 10 is a template 20, which can be made of a thickAlN layer. An AlN/AlGaN superlattice can also be included in template 20for strain management. Following template 20 is a thick n-AlGaN layer 25for electron supplier and ohmic contact formation. Layer 27 formed overlayer 25 is a lightly doped n-AlGaN layer for current spreading,preparing uniform current injection into the following AlGaN/AlGaN MQWactive-region 30. A vertically conductive UV transparent p-type AlGaNlayer structure 40 is formed on active-region 30. In this embodimentshown in FIG. 3, p-AlGaN layer structure 40 includes a smooth electronblocking p-AlGaN layer 41, a rough p-AlGaN layer 42, with vanishing base420, a stack of three pairs of layers 43/44, again with vanishingportions 432. Here vanishing portions 432 means that very littleportions 432 are formed during the epitaxial growth of layer 43, layer43 is formed mainly as nano-dots in the bottom parts of depressions 422,and these nano-dots may or may not be connected to each other byportions 432 or thin layer portion of layer 43, or some of the nano-dotsare isolated nano-dots and some of the nano-dots are connected to eachother via portions 432. A UV light reflective p-type ohmic contact 51 isformed over p-AlGaN layer structure 40. The ohmic contact property isassured by forming the p-type ohmic contact 51 in contact with thehighly conductive three-dimensionally confined portions 431. In anembodiment, the nominal layer thickness of layers 41, 42, 43, and 44 canbe 30, 50, 2, and 8 nm, respectively. A UV light reflective n-type ohmiccontact 61 is formed over layer 25, and p− and n− contact pads 52 and 62are respectively formed over reflective ohmic contacts 51 and 61. Whenforward bias is applied to the device shown in FIG. 3, hole currenttunnels through the vertically aligned nano-dot portions 431, providingsufficient hole injection into active-region 30. In another embodiment,a thin p-type GaN or InGaN layer of thickness not larger than 5 nm canbe inserted in-between p-AlGaN layer structure 40 and p-type ohmiccontact 51.

The arrays of vertically aligned nano-dot portions 431 shown in FIG. 2and FIG. 3 are formed via physical location confinement by depressions422 formed on the rough surface of layer 42. According to another aspectof the present invention, arrays of vertically aligned nano-dot portionscan also be formed via biaxial compressive strain confinement. Shown inFIG. 4 is another vertically conductive UV transparent p-AlGaN layerstructure 40′, with compressive strain confined vertically alignednano-dot portions 431′ as the hole conducting channels. Layer 41,together with layers 44′, exerts large biaxial compressive strain tonominal layer 43′. The compressive strain experienced by nominal layer43′ should be larger than 0.7%, preferably larger than 1.0%. That is tosay, if nominal layer 43′ is made of Mg-doped GaN, layer 41 and layer44′ can be Mg-doped AlGaN with Al-composition larger than 30%,preferably larger than 40%, in view of the lattice constant differencebetween GaN and MN. Al-composition of layer 41 can be higher than thatof layer 44′ by 0-10%, such as 5-10%. According to this aspect of thepresent invention, layer 43′ is preferably made of Mg-doped InGaN withIn-composition in the range of 0-10% such as 5-10%. The incorporation ofIndium into layer 43′ enlarges the compressive strain experienced bylayer 43′ hence facilitates the formation of nano-dot portions 431′. Thenominal thickness for nominal layer 43′ is within the range of 1-5 nm.The so-designed compressive strain will induce epitaxial growth of layer43′ on layer 41 or 44′ to follow the Stranski-Krastanov (SK) growthmode. According to the SK growth mode, under a critical thickness (forexample under 1 nm or under 0.6 nm) layer 43′ will firstly undergotwo-dimensional layer-by-layer growth, and this wetting process leads tothe formation of thin layer or wetting layer portions 432′. Above thecritically thickness, growth of layer 43′ transforms fromtwo-dimensional layer-by-layer growth to three dimensional islandgrowth, resulting in the formation of nano-dot portions 431′. Overgrowthof nominal layer 43′ can result in island enlargement and coalescence,losing the benefit of nano-dots for harvest of high concentration ofholes arising from the three-dimensional valence band and polarizationdiscontinuities. Depending on the compressive strain under interest ofthe present invention, the wetting layer portions 432′ are of thicknessin the range of 0.6-1.0 nm. The density and size of the nano-dotportions 431′ are controlled by formation temperature and nominal layerthickness of layer 43′. According to an embodiment of the presentinvention, the sheet density of nano-dot portions 431′ is in the rangeof approximately 10¹⁰ to 10¹² cm⁻². Portions 431′ preferably possess alateral dimension in the range of 2-20 nm such as 5-15 nm, and avertical dimension in the range of 1-5 nm such as 1-3 nm.

Further refer to FIG. 4, layer 43′ and layer 44′ can be repeatedlystacked on top of each other for more than one time. For example, layers43′/44′ can be stacked for 3 times to 10 times. Repeatedly alternativelystacking layer 43′ and layer 44′ leads to the formation of arrays ofvertically aligned nano-dot portions 431′. By controlling formationconditions, nano-dot portions 431′ in different layers 43 can besubstantially vertically aligned. It is also possible to form layers 43,where nano-dot portions 431′ in different layers 43 are not verticallyaligned. When a vertical forward bias voltage is applied to p-AlGaNlayer structure 40′, the otherwise uniformly distributed electric fieldwill be redistributed, intensified and focused on the highly conductivenano-dot portions 431′, enabling hole tunneling current within thevertically aligned highly conductive nano-dot portions 431′. That is tosay, p-AlGaN layer structure 40′ is a vertically conductive UVtransparent p-type AlGaN layer structure.

According to one embodiment of this aspect of the present invention,MOCVD is used to epitaxially form vertically conductive UV transparentp-AlGaN layer structure 40′ shown in FIG. 4. Layers 41 and 44′ areformed at a temperature of 1000-1050° C., and layers 43′ are formed at atemperature of 800-1000° C., in order to achieve the desire sheetdensity of nano-dot portions 431′, the lower the formation temperature,the higher the sheet density of nano-dot portions 431′.

FIG. 5 illustrates a UV light emitter device structure employed withvertically conductive UV transparent p-AlGaN layer structure 40′.Substrate 10 can be selected from sapphire, AlN, SiC, Si, and the like.Formed over substrate 10 is a template 20, mainly made of a thick AlNlayer. An AlN/AlGaN superlattice can also be included in template 20 forstrain management. Following template 20 is a thick n-AlGaN layer 25 forelectron supplier and ohmic contact formation. Layer 27 formed overlayer 25 is a slightly doped n-AlGaN layer for current spreading,preparing uniform current injection into the following AlGaN/AlGaN MQWactive-region 30. A vertically conductive UV transparent p-AlGaN layerstructure 40′ is formed over active-region 30. In this embodiment shownin FIG. 5, p-AlGaN layer structure 40′ includes a smooth electronblocking AlGaN layer 41, a stack of four pairs of layers 43′/44′. A UVlight reflective p-type ohmic contact 51 is formed over p-AlGaNstructure 40′. The ohmic contact property is assured by forming thep-type ohmic contact 51 in contact with the highly conductivethree-dimensionally confined portions 431′. In some embodiments, thenominal layer thickness of layers 41, 43′, and 44′ can be 50, 2, and 8nm, respectively. A UV light reflective n-type ohmic contact 61 isformed over layer 25, and p− and n− contact pads 52 and 62 arerespectively formed over reflective ohmic contacts 51 and 61. Whenforward bias is applied to the device shown in FIG. 5, hole currenttunnels through the vertically aligned nano-dot portions 431′, providingsufficient hole injection into active-region 30. In another embodiment,a thin p-type GaN or InGaN layer of thickness not larger than 5 nm canbe inserted in-between p-AlGaN layer structure 40′ and p-type ohmiccontact 51.

The light emitting devices made according to FIG. 3 and FIG. 5 aresubstantially UV transparent, allowing improved UV light extractionefficiency.

The present invention has been described using exemplary embodiments.However, it is to be understood that the scope of the present inventionis not limited to the disclosed embodiments. On the contrary, it isintended to cover various modifications and similar arrangement orequivalents which can be obtained by a person skilled in the art withoutcreative work or undue experimentation. The scope of the claims,therefore, should be accorded the broadest interpretation so as toencompass all such modifications and similar arrangements andequivalents.

What is claimed is:
 1. A method of forming a vertically conductive UVtransparent p-type layer structure, comprising steps: forming a smoothp-type AlGaN layer on a light emitting active-region of a light emittingdevice via metalorganic chemical vapor deposition at a temperaturebetween 1000-1050° C., wherein the smooth p-type AlGaN layer serves asan electron blocking layer; forming a rough p-type AlGaN layer on thesmooth p-type AlGaN layer via metalorganic chemical vapor deposition ata temperature 50-200° C. lower than that of the smooth p-type AlGaNlayer so as to form surface protrusions of height 5 to 30 nm, lateralsize 10 to 100 nm, and sheet density 10¹⁰ to 10¹² cm⁻² on the roughp-type AlGaN layer, wherein the smooth p-type AlGaN layer and the roughp-type AlGaN layer are UV transparent for wavelength below 320 nm;forming a p-type InGaN layer with an In-composition of 0-10% or a p-typeAlGaN layer with an Al-composition of 0-10% on the rough p-type AlGaNlayer via metalorganic chemical vapor deposition at a temperaturebetween 800-1000° C., such that a layer of nano-dots is formed with thenano-dots being formed within depressions between the protrusions of therough p-type AlGaN layer, wherein the nano-dots possess a sheet densityof 10¹⁰ to 10¹² cm⁻², a lateral dimension of 2-20 nm, a verticaldimension of 1-5 nm; and conformally forming a confining p-type AlGaNlayer on the p-type InGaN layer or the p-type AlGaN layer viametalorganic chemical vapor deposition to cover the nano-dots.
 2. Themethod of forming the vertically conductive UV transparent p-type layerstructure according to claim 1, wherein an Al-composition of theconfining p-type AlGaN layer is substantially the same as that of therough p-type AlGaN layer.
 3. The method of forming the verticallyconductive UV transparent p-type layer structure according to claim 1,wherein a formation ambient in the metalorganic chemical vapordeposition for forming the rough p-type AlGaN layer is ammonia-nitrogenambient.
 4. The method of forming the vertically conductive UVtransparent p-type layer structure according to claim 1, wherein anAl-composition of the smooth p-type AlGaN layer is 5-10% higher than anAl-composition of the rough p-type AlGaN layer.
 5. The method of formingthe vertically conductive UV transparent p-type layer structureaccording to claim 1, wherein multiple pairs of the confining p-typeAlGaN layer and the layer of nano-dots are formed on the rough p-typeAlGaN layer.
 6. The method of forming the vertically conductive UVtransparent p-type layer structure according to claim 1, furthercomprising: forming a UV light reflective p-type ohmic contact on theconfining p-type AlGaN layer.
 7. The method of forming the verticallyconductive UV transparent p-type layer structure according to claim 1,wherein a thickness of the smooth p-type AlGaN layer is in a range of5-30 nm.
 8. The method of forming the vertically conductive UVtransparent p-type layer structure according to claim 1, wherein athickness of the confining p-type AlGaN layer is in a range of 2-10 nm.9. A method of forming a vertically conductive UV transparent p-typestructure, comprising steps: forming a smooth p-type AlGaN layer on alight emitting active-region of a light emitting device via metalorganicchemical vapor deposition at a temperature of 1000-1050° C.; forming ap-type AlGaN layer on the smooth p-type AlGaN layer via metalorganicchemical vapor deposition at a temperature of 1000-1050° C., wherein theAl-composition of the smooth p-type AlGaN layer is 5-10% higher than anAl-composition of the p-type AlGaN layer; forming a p-type InGaN layerwith an In-composition of 0-10% and a nominal thickness of 1-5 nm on thep-type AlGaN layer in Stranski-Krastanov growth mode via metalorganicchemical vapor deposition at a temperature of 800-1000° C., so that thep-type InGaN layer is formed with a wetting layer portion and nano-dots,wherein the nano-dots possess a sheet density of 10¹⁰ to 10¹² cm⁻², alateral dimension of 2-20 nm, a vertical dimension of 1-5 nm.
 10. Themethod of forming the vertically conductive UV transparent p-type layerstructure according to claim 9, further comprising: forming a UV lightreflective p-type ohmic contact on the p-type InGaN layer.
 11. Themethod of forming the vertically conductive UV transparent p-type layerstructure according to claim 9, wherein a thickness of the p-type AlGaNlayer is in a range of 2-10 nm.
 12. The method of forming the verticallyconductive UV transparent p-type layer structure according to claim 9,wherein a thickness of the smooth p-type AlGaN layer is in a range of5-30 nm.